1. Field of the Invention
This invention relates to a means and method for improved pneumatic comminution, in particular, a means and method of pneumatically comminuting various materials with improved control and results.
2. Problems in the Art
Comminution, the pulverization or breaking apart of materials into small parts, is a significant operation in many industries, particularly the coal and cement industries which require tremendous amounts of crushing and grinding. However, current comminution technology used by industry is both energy intensive and inefficient. Annual electrical energy consumption in size reduction operations by United States industry is approximately 32 billion kilowatt hours (KWH). More than half of this energy is consumed in the crushing and grinding of minerals. An additional 3.7 billion KWH per annum is contained in energy inconsumables, such as grinding media and liners. The total amount of energy approaches 2% of the national electric power production.
The amount of energy used by United States industry to produce its products not only contributes to that cost of production, but also is a factor in the end products' marketability on world markets. A study of United States industries reveals that the cost of a commodity intended for both national and international markets is closely associted with the cost of energy required to manufacture that commodity. These energy costs are particularly high in the primary metals, chemical, food, paper and petroleum industries All of these industries rely heavily upon particle size reduction operations, which is therefore a significant contribution to product cost. It can therefore be seen that there exists a continuing need for improvements in comminutors, and energy consumption in the comminution process Two vivid examples are the coal and cement industries.
It is well known that using coal as an energy source presents several barriers preventing its widespread use. Among these are derating of a boiler burning natural gas or oil, more elaborate handling and combustion facilities, and expensive pollution control. Investigations concerned with coal combustion and pollution control show promise of removing these barriers without significant cost increases. Thus, the price of coal should remain favorable and yield widespread usage of coal.
It is known in the art that micronized coal burns more efficiently than lump coal. Micronized coal is lump coal which is disintegrated to micron sized particles. Micronized coal also provides for easier handling, more efficient, complete and controllable combustion, and an opportunity to reduce particulate emissions.
A micronized coal particle has a larger surface per unit volume, thereby increasing the burning rate. Micronized coal burns much like a No. 2 oil, suggesting that retrofitting can be accomplished by replacement of the oil or gas burner with a coal burner, and derating of a furnace is unnecessary. Further development of techniques for combustion systems using micronized coal and applications of these techniques to industrial size furnaces is in process.
It is interesting to note that studies have shown that the critical pollution problem involved with the sulfur content in coal can be controlled or eliminated by injecting limestone into the coal during the combustion process. The calcium reacts with the sulfur to produce calcium sulfate particles which are removed with the ash, using conventional particle gas separators. To facilitate injection of the limestone, it too must be micronized. The combined micronized limestone and coal represents a viable method of reducing both energy costs, through use of coal, and sulfur dioxide pollutants by the sulfur calcium reaction. There is therefor a continuing need for an apparatus which will allow efficient and economical coal micronization.
Micronized coal of the size between 5 micrometers (.mu.m) and 30 .mu.m is more advantageous than the particles produced by conventional pulverizers where particle sizes range from 50 to 150 .mu.m. The centrifugal comminutor of this invention will efficiently and economically produce coal particles between 5 .mu.m and 30 .mu.m in diameter.
A second major advantageous use for comminution exists in the cement industry In the cement industry, the surface area per unit weight has become a standard for characterizing cement quality. Acceptable fineness is around 3,200 to 5,200 cm.sup.2 per gram (cm.sup.2 /gm) of cement. This measurement, known as Blaine Surface Measurement, is made by measuring the pressure drop which results from the flow of air through a standard packed bed of cement.
Recent studies have shown that the particle size of cement is important, based upon the following findings (1) by controlling the cement particle size to below 20 .mu.m, with a Blaine area of only 2600 cm.sup.2 /gm, strengths equaling that of normally ground cements of 3600 cm.sup.2 /gm Blaine area can be achieved; (2) The amount of ground clinker in a 2.5 .mu.m particle size range has large effects on bleeding, water requirements for flow, and strength of development; (3) Controlled product particle size of cement grinding results in cements of as high or higher strengths at ages from 1-60 days at Blaine areas of 450-800 cm.sup.2 /gm, substantially lower than the normal grinds of the same composition.
It is estimated that the adoption of particle size control in clinker grinding by the entire United States cement industry would result in a 27% saving in grinding energy, and an 8.5% savings in kiln fuel. To achieve such control, however, reliable on-line (real time) particle size and specific surface measurement devices need to be developed. The centrifugal comminutor of this invention can be success fully used in the cement industry.
It is generally believed that high specific surface areas produce high strength cement. The actual particle size distributions also influence cement strength. The particle sizes that have the greatest effect of cement strength are 5 to 30 .mu.m.
By comminuting the elements of cement, namely, limestone and clinker in the comminutor of this invention, improvements in cement quality and savings in energy consumed in producing cement can be achieved.
In other areas too, besides coal and cement, a tremendous energy savings could be realized by reducing energy consumption for other comminuted products.
For example, comminution is utilized on a significant scale for many other commodities including, but not limited to, the following: aluminum, arsenic, asbestos, barite, boron, calcium, ceramics, chromium, clays, copper, diatomite, feldspar, fluorspar, golds, grain, gypsum, iron ore, lead, lithium, magnesium, manganese, mercury, mica, molybdenum, nickel, perlite, phosphate, potassium, pumice, rae earth, sand and gravel, salts, silicon, silver, a stone, chalk, titanium, tungsten, uranium, vermiculite, and zinc. It is estimated that the energy used for comminution of these materials approaches 33 billion kilowatt hours per year.
Existing technology utilizes such apparatuses as ball mills, rod mills, roll mills, autogenous mills, and hammer mills as fine grinders; and attrition and fluid energy mills as ultrafine grinders. The tremendous cost of these devices centers not only on their operating energy consumption, but also on their capital costs, maintenance, metal loss from attrition of moving parts with the material being comminuted, and ancillary equipment which is needed to operate in conjunction with these devices.
The present invention represents a significant improvement over the above mentioned conventional comminutors as it utilizes pneumatics and particle-to-particle attrition for both input and output conveyance of the material and comminution of the material, respectively.
Pneumatic or vacuum comminution, was the subject of U.S. Pat. No. 3,255,793, issued to Clute on June 14, 1966. Clute attempted to utilize pneumatic comminution for crop grinding. Clute used a vertically rotating fan in a housing having an horizontal inlet along the horizontal fan axis. However, Clute neither encountered nor contemplated the use of this device for coal comminution or cement industry applications nor was the Clute device successful in its intended use. Furthermore, it has been found that Clute's invention was and is not successful because of problems with the pneumatics and because of excessive and unacceptable metal loss from the blades of fans. The device of this invention accomplishes much smaller size reduction than Clute when comparable energy is expended, with significantly reduced wear.
In the centrifugal action of the present improved comminutor, its non-uniform acceleration of various massed particles causes particle-to-particle attrition of the material in the area directly before the fan (called the rotational impact zone or RIZ). Thus, comminution is substantially achieved prior to the particles passing through the rotary fan. As a result, metal wear is lessened considerably.
The present application, a continuation-in-part from U.S. Ser. No. 697,042, improves further on that co-pending application by further improving the control of the air flow through the comminutor and the action of the particles being comminuted to produce more accurate and uniform final product particle size.